1. Field of the Invention
The present invention relates to a cylindrical roller bearing suitable for supporting a shaft in high speed installations such as machining tools, jet aircraft engines, and gas turbines.
2. Description of the Related Art
Spindle apparatuses of machining tools such as machining centers, CNC lathe turning machines, milling machines, and the like are often operated at high speed for the sake of better machining efficiency and higher precision of work, and the recent trend is toward further increase in the speed of spindle rotation.
Generally, in a spindle apparatus of a machining tool, the spindle is supported rotatably relative to the housing by rolling bearings respectively disposed at the front side or tool side and at the rear side opposite the tool side. Lubrication of the rolling bearings is achieved by any of oil mist lubrication, air/oil lubrication, jet lubrication, and grease lubrication, in accordance with various conditions of use. The rolling bearing on the front side normally has a structure that does not allow any axial displacement of the spindle and so is a “fixed side”, while the bearing on the rear side has a structure that allows for some axial displacement of the spindle for absorbing or relieving expansion of the spindle in the axial direction due to heat generated during the operation, and thus this side is a “free side.” Duplex angular ball bearings, or duplex angular ball bearings with double row cylindrical roller bearings are commonly used as the roller bearing on the front side, while duplex angular ball bearings, or double row or single row cylindrical roller bearings are commonly used as the roller bearing on the rear side.
A cylindrical roller bearing generally includes an inner ring having a raceway on its outer periphery, an outer ring having a raceway on its inner periphery, a plurality of cylindrical rollers rotatably arranged between the raceways of the inner and outer rings, and a cage for holding the cylindrical rollers at circumferentially spaced locations.
The inner ring may be provided with collars on both sides, in which case there are provided recesses at respective corners where the collar surface of both collars and inner raceway intersect each other. These recesses are formed as a result of providing an undercut when grinding the raceways and the collar surfaces. Similarly, at the corners where the rolling surfaces and both end faces of the cylindrical rollers intersect, there are provided respective chamfers. The distance between the axially opposing collar surfaces is slightly larger than the length of the cylindrical rollers, so that guide clearances are secured between the cylindrical rollers and the collars.
Because the rolling surfaces of the cylindrical rollers and the raceways of the inner and outer rings make line contact with each other, the cylindrical roller bearings can hold heavy radial loads and are suitable for high speed applications. On the other hand, more heat is generated during high speed rotation in these bearings as compared to ball bearings, and the portions where the cylindrical rollers and the collars make sliding contact are particularly susceptible to heat generation and wear. That is, because the guide clearances mentioned above allow for some freedom of inclination of the cylindrical rollers, the occurrence of “skew” is inevitable, in which the axial line of the cylindrical rollers inclines relative to the axial line of the bearing during rotation. The angle of the axial line of the cylindrical rollers relative to the axial line of the bearing is named as a skew angle θ. When the cylindrical roller is skewed, an axial component is created in the drive force given by the rotating raceway, which will act as an axial thrust F on the cylindrical roller, pressing its end toward the collar on one side. This can cause high friction resistance in the sliding contact portions, resulting in heat generation and wear.
Various improvements have been suggested so far with respect to such problem. Japanese Patent Publication No. Sho 58-43609, for example, shows a bearing construction in which the height of the recesses is made higher than the height of chamfers of the cylindrical rollers, and in which the collar surfaces are provided with an increasing taper at a predetermined angle toward the outside in axial direction, whereby lubrication state of the sliding contact portions is improved.
Japanese Patent Laid-Open Publication No. Hei 7-12119 shows a bearing construction in which, when the cylindrical roller is skewed, its outer peripheral portion on either end face will contact the collar surface at a location toward the base end, so that the edge load on the sliding contact portion is reduced as compared to the case in which the outer peripheral portion on either end face of the cylindrical roller makes contact with a distal edge of the collar. In the case when both ends of the cylindrical rollers respectively make contact with the collars on both sides, the skew angle becomes maximum, that is called maximum skew angle θMAX.
The cylindrical rollers are free to incline by the amount of the guide clearances as noted above, and during rotation of the bearing, they rotate and revolve while changing their attitude from one second to the next within the range of maximum skew angle θMAX.
Referring now to the model view of FIG. 7, when the cylindrical roller 23 is skewed at a skew angle θ lower than the maximum skew angle θMAX it is pressed by the axial thrust F mentioned above toward one side in the axial direction, and rolls in a state in which it is pressed against one of the collars 21b. With referring to FIGS. 8 and 17, the skew angle θ is the angle of the axial line lcr of the cylindrical rollers 23 relative to the axial line 1b of the bearing 21. In the case when a boundary R13 between the end face 23b and the chamfer 23c of the cylindrical roller 23 makes contact with a boundary R11 between the collar surface 21b1 and the recess 21c, the skew angle θ is called a critical skew angle θT. With referring to FIG. 18, in the case when both ends of the cylindrical rollers 23 respectively make contact with the collars 21b on both sides, the skew angle θ becomes the maximum skew angle θMAX. The state of contact between the cylindrical roller 23 and the collar 21b changes as follows in accordance with the skew angle θ in the range of θ<θT<θU<θMAX.
In the range of 0<θ<θT, a boundary R13 between the end face 23b and the chamfer 23c of the cylindrical roller 23 makes contact with a boundary R11 between the collar surface 21b1 and the recess 21c as indicated by a black circle in FIG. 8, while, in the range of θT<θ<θU, the boundary R13 between the end face 23b and the chamfer 23c of the cylindrical roller 23 makes contact with the collar surface 21b1 as indicated by a black circle in FIG. 9. As the skew angle θ approaches θU, the boundary R13 between the end face 23b and the chamfer 23c of the cylindrical roller 23 makes contact with a boundary R12 between the collar surface 21b1 and the chamfer 21b3 (not shown). The maximum skew angle θMAX will have been reached when both end portions of the cylindrical roller 23 respectively make contact with the collars 21b on both sides (not shown).
FIG. 10 shows the relationship between the skew angle θ of the cylindrical rollers 23 and the contact surface pressure P between the cylindrical rollers 23 and the collars 21b in solid line, and the relationship between the skew angle θ and the axial thrust F acting on the cylindrical rollers 23 in broken line. As can be seen from the drawing, the axial thrust F increases in proportion to the skew angle θ.
The contact surface pressure P increases rather drastically with the increase of the skew angle θ in the range of 0<θ≦θT. This is because the cylindrical roller 23 and the collar 21b make contact with each other at the boundaries R13 and R11 as shown in FIG. 8, and because the axial thrust F becomes larger with the increase of the skew angle θ. It was ascertained through tests that, in the range of θ0≦θ≦θT indicated by cross hatching in FIG. 10 in particular, the contact surface pressure P exceeds a certain level P0 at which wear occurs in the contact portions.
After the skew angle θ exceeds θT, the surface contact pressure P decreases below the level P0 and moves stably within a relatively low range irrespective of the increase of the skew angle θ. This is because the state of contact between the cylindrical roller 23 and the collar 21b has changed from the state of contact at the boundaries R13 and R11 shown in FIG. 8 to the state of contact between the boundary R13 and the collar surface 21b1 shown in FIG. 9.
As the skew angle θ approaches θU, the contact surface pressure P increases abruptly and exceeds the level P0 at the time point when it reaches the value θU. This is because the state of contact between the cylindrical roller 23 and the collar 21b has changed from the state of contact between the boundary R13 and the collar surface 21b1 shown in FIG. 9 to the state of contact at the boundaries R13 and R12.
As described above, the contact surface pressure P between the cylindrical rollers and the collars is beyond the level P0 at which wear occurs in the contact portions before the skew angle reaches its maximum level θMAX, namely in the ranges of θ0≦θ≦θT and θU≦θ<θMAX, and this is considered to be a substantial factor in causing heat generation and wear in the contact portions.
There is no mention of the phenomenon described above in Japanese Patent Publication No. Sho 58-43609 mentioned above and so it is not directed to resolve this problem. Japanese Patent Laid-Open Publication No. Hei 7-12119 mentioned above merely defines a contact state between outer peripheral portions on either end face of the cylindrical rollers and the collar surfaces at a maximum skew angle θMAX. It does not refer to, nor does it provide a solution, to the problem arising before the skew angle reaches its maximum level θMAX.
In a construction in which duplex angular ball bearings are used on the rear side, clearances are given between the outer ring of the bearing and the housing so as to allow sliding displacement therebetween, because the bearings themselves cannot accommodate axial displacement of the spindle. Alternatively, a sliding member such as a ball bush may be interposed between the outer ring and the housing may lead to creep and wear in the contact portions therebetween particularly when the rotation speed of the spindle is high. In the latter construction, the provision of additional sliding members such as ball bushes results in larger numbers of components and assembling steps. Furthermore, the problem common to both constructions is that costs tend to be high because of the use of angular ball bearings which require two bearings in combination and which often use ceramic balls to be suitable for higher speed applications. Thus the spindle apparatus could be improved in this respect for achieving cost reduction.
Cylindrical roller bearings, on the other hand, can advantageously be used for the rear side for resolving the above problems, because axial displacement of the spindle can be absorbed or relieved by sliding displacement between the cylindrical rollers and the raceways. Moreover, cylindrical roller bearings can hold heavy radial loads because the cylindrical rollers and the raceways make line contact, and therefore they are preferable in securing necessary rigidity of the spindle. On the other hand, cylindrical roller bearings have the problem that more heat is generated during high speed rotation as compared to angular ball bearings.
Driving system for spindle apparatus has recently changed from the belt drive system to a built-in motor drive system in which the spindle is rotated at high speed by a built-in motor, and the latter is now the mainstream because of the demands for higher speed and efficiency. In a spindle apparatus with this drive system, however, there tends to be a difference in the temperature between the spindle and the housing due to the built-in motor which generates heat, because of which the negative clearances in the rolling bearings are increased as compared to an apparatus with the belt drive system, i.e., they are operated in a pre-loaded condition. There is also the problem that thermal expansion of the spindle in the axial direction tends to be large because of the long span between the rolling bearings on the front side and on the rear side.